Past studies of bronchopulmonary resistance to bacterial infection have focused principally on the alveolar macrophages phagocytic system. This is the resident phagocytic system which represents a nonspecific cellular defense mechanism responding to the alveolar deposition of organisms and other particles. However, a second phagocytic system, the polymorphonuclear leukocyte, often also responds early to the presence of organisms invading the lung. These inflammatory cells constitute a very potent phagocytic system which is recruited into the lungs; however, their contribution to the overall defenses of the lung have not been clearly established. Using mthodology to separate the lung cell systems and concurrently quantitate the participation of both macrophages and polymorphonuclear leukocytes in bacterial ingestion and killing, this project will examine and establish: a) the basic cellular defense mechanisms that each of these two phagocytic systems constitute individually and how they interact to provide resistance to bacterial challenges of the lung; b) how these defense mechanisms are altered with the development of immunity during the course of infection resulting in resolution of the infection; and c) how these defense mechanisms are expressed during bacterial challenge of the resistant immune lung.